An electroacoustic transducer generally comprises two comb-like electrodes arranged on a piezoelectric substrate, with interlocking fingers, generally arranged on a periodic grid. An electrical signal applied to the electrodes excites an acoustic surface wave if the signal frequency corresponds to the period of the finger structure.
The characteristics of an electroacoustic transducer are essentially predetermined by the number, width, connection sequence and longitudinal position (i.e., position along the propagation of the acoustic wave) of the fingers, as well as by the aperture (i.e., the length of the active overlap region of the adjacently arranged fingers of different electrodes). These are generally selected so that, if possible, only one acoustic vibration mode is excited, against which the design is optimized in terms of said variable parameters.
Electroacoustic transducers for surface wave components are especially used in the development of reactance filters.
Single-gate resonators are known, for example, that include a transducer delimited on one side by reflectors. Also known are DMS filters (Double Mode Surface Acoustic Wave Filter) and multiple gate resonators with several transducers that are acoustically connected to one another. Filters with SPUDT transducers (Single Phase Unidirectional Transducer) with a preferred emission direction of the acoustic wave are also known.
Also known are weighted transverse filters that include transducers with a transverse overlapping profile of the electrode fingers. In this context, the overlapping profile is utilized to structure the impulse excitation within the time range, which is ideally to deliver a rectangular pass-through band of the transmission function.
An important parameter of the reactance filters is the input attenuation, which corresponds to the maximum attenuation of a signal passing through the filter in the pass-through band. Anything that increases the input attenuation worsens the transmission characteristics of the overall system, so that the lowest possible losses are also considered desirable in this case.
For this reason, transducers or resonators that are used in the reactance filters should, have the largest possible real component of the input admittance at their resonant frequency. In previously known transducers, however, a portion of the energy of the acoustic wave is lost, especially because the almost rectangular excitation profile in the transducer diverges from the existing energy density profile of the wave, so that as a result of poor fit of the excitation to the actual energy distribution, the electrically excited acoustic wave can only be partially converted to an electric signal, which is why signal losses occur.
The (lateral or transverse) profile of a physical variable describes the distribution of this variable as a function of the corresponding local coordinates, the x and y axes respectively being selected as the lateral and transverse directions. The energy density profile is here particularly defined as the decrease in energy density in boundary regions compared with a centrally located region.
The energy density profile in a transducer or resonator can be determined by means of power compatibility measurements, for example, where the migration of the electrode material can be a measure of energy density at a given location.
It is known that the longitudinal excitation profile can be adjusted by transverse weighting of the aperture, the aperture being selected to be at its maximum in the center of the transducer and decreased toward the exterior. However, such weighting of the aperture reduces the active region in which the acoustic wave is excited. Moreover, the energy density profile, which is partly attributable to edge effects, is emphasized in the transverse direction, because the longitudinal length of a transducer which is configured as a resonator is generally considerably greater than its transverse length. The edge effects and the associated losses play an increasingly important role as apertures become smaller and smaller.
It is possible to adjust the transverse excitation profile, i.e., the amplitude A(y) of the acoustic wave, which depends on the transverse coordinate y, to the transverse energy density profile E(y) by tapering the fingers toward the bus bar so that the amplitude of the excited wave in the edge region decreases in comparison with the centrally located active region. The disadvantage of this solution is that the finger resistance increases with the smaller finger width. Furthermore, the minimum width of the metal structures is technologically limited, which is especially problematic with transducers or resonators designed for higher frequencies.
Another possibility for adjusting the excitation profile to the energy density profile consists of making the fingers wider in the direction of the bus bar, which would however lead to the excitation of damaging bulk waves and therefore to an increase in losses.